This invention relates to catalytic dehydrogenation of alkylaromatic compounds to produce vinylaromatic compounds. More particularly, this invention relates to a method and apparatus in which the alkylaromatic feed is first vaporized and then passed in heat exchange relation with effluent gases from the dehydrogenation zone before the feed is introduced into the dehydrogenation zone. Specifically, this invention relates to a method and apparatus for producing vinylaromatic monomers by catalytic dehydrogenation of alkylaromatic compounds wherein the alkylaromatic feed is flash vaporized.
Vinylaromatic compounds such as styrene, vinyltoluene, alpha-methylstyrene, divinylbenzene and the like are important as monomers from which useful polymers are made. These monomers are typically produced by catalytic dehydrogenation of alkylaromatic compounds to the corresponding vinylaromatic compounds in the presence of steam at elevated temperatures. Before the alkylaromatic feed is introduced into the dehydrogenation zone, it is typically preheated by first passing it in indirect heat exchange relation with steam and thereafter passing it in indirect heat exchange relation with hot effluent gases withdrawn from the dehydrogenation zone.
Although some vaporization of the alkylaromatic feed may take place during the initial heat exchange operation with steam, vaporization of the feed occurs primarily in the feed/effluent heat exchanger. Generally, the feed comprises, in addition to the desired alkylaromatic compound, minor proportions of vinylaromatic compound. This creates a problem in the feed/effluent heat exchanger because the alkylaromatic compound tends to vaporize first, leaving behind a liquid phase with an enriched content of vinylaromatic compound. These vinylaromatic compounds are highly reactive and tend to polymerize in the liquid phase at temperatures encountered in the feed/effluent heat exchanger. The preferential evaporation of alkylaromatic compound and resulting enrichment of the remaining liquid phase in vinylaromatic compound increases the likelihood that vinylaromatic compound molecules in the liquid phase will encounter each other and polymerize. As a result, vinylaromatic polymer deposits form in the feed/effluent heat exchanger. Polymerization occurs when liquid phase material containing an enriched amount of vinylaromatic compound contacts the hot tube walls of the feed/effluent heat exchanger which typically have a temperature lying in the range from about 500 to about 600 degrees C.
Carbonaceous deposits which progressively accumulate on the catalyst in the dehydrogenation zone may be removed by periodic steaming of the catalyst. Such treatments, however, do nothing to ameliorate the formation of polymer and/or coking of the feed/effluent heat exchanger. After a period of operation, it becomes necessary to take the feed/effluent heat exchanger out of service for cleaning. As a result, the cost of producing vinylaromatic monomers is increased both by the increased operating cost attributable to the cleaning operation and by the loss in productivity of the equipment.
The problem of polymer formation and coking in the inlet system leading to the dehydrogenation reactor is particularly severe in the dehydrogenation of ethyltoluene to produce vinyltoluene. When ethyltoluene is being dehydrogenated to produce vinyltoluene, it is necessary to clean the heat exchanger at from 1 to 6 month intervals.